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STORY: The U.S. Securities and Exchange Commission is investigating whether recent stock sales by Tesla Chief Executive Elon Musk and his brother Kimbal Musk "violated insider trading rules," the Wall Street Journal reported Thursday.

According to the report, the investigation began last year after Kimbal Musk sold shares of the electric carmaker valued at $108 million - a day before brother Elon Musk polled Twitter users asking whether he should offload 10% of his stake in Tesla.

Ten days after Musk’s poll – on Nov. 16 - the SEC issued a subpoena seeking information related to some financial data.

The potential probe would escalate Musk’s battle with regulators as they scrutinize his social media posts and Tesla's treatment of workers, including accusations of discrimination.

Just last week, Tesla and Musk accused the SEC of harassing them with an "endless" and "unrelenting" investigation to punish Musk for being an outspoken critic of the government.

Tesla’s stock has fallen about 33% since Musk began selling billions of dollars’ worth of shares on Nov. 8, two days after the poll, in which 58% of voters supported his proposal to offload shares.